More specifically, the piezocone is adapted to perform static penetrometric tests.
Static penetrometric testing has, in recent years, become the most widespread method of obtaining in situ information on the stratigraphic and characteristic properties of soil deposits. In particular, the static penetrometric test with measurement of interstitial pressures (hereinafter called piezocone test), although introduced relatively recently, has already become a standard test in the range of possible in situ geotechnical tests, thanks to its relative simplicity, low cost and wealth of available experience. The possibility of measuring interstitial pressures in the soil during the test significantly improves the quality of the stratigraphic profile obtained from a static penetrometric test because it allows identifying even thin soil lenses and is more accurate in estimating the mechanical parameters.
The piezocone acts as a probe which measures the resistance of the soil when the piezocone is pushed into it.
One type of piezocone which may be used in stratigraphic analysis extends along an axis of extension.
The piezocone includes a pressure sensor unit to measure the interstitial pressure of the soil. The interstitial pressure of the soil, also called “interstitial overpressure” develops when the piezocone is pushed into the soil. The sensor unit comprises a pressure transducer inside the piezocone.
The pressure transducer may operate, for example, by deflection of a blade.
The sensor unit comprises a porous annular element designed to prevent agglomerates or particles of excessive size from finding their way into the sensor unit.
The lateral outside surface of the porous annular element defines a surface portion of the piezocone situated around the axis of extension of the piezocone itself.
To cause the interstitial pressure of the soil to be propagated as far as the pressure transducer, the sensor unit comprises a duct inside the piezocone, interposed between the porous annular element and the transducer so that the transducer can measure the pressure applied to the aforesaid surface portion. As mentioned above, this surface portion coincides with the lateral outside surface of the porous annular element. During use of the piezocone, the sensor unit contains a working liquid, which may be different from the interstitial liquid which is present in the soil and whose pressure, known as interstitial pressure or soil pore pressure, is to be measured. The working liquid is at least partly in equilibrium with the interstitial liquid so that variations in the pressure of the interstitial liquid produce corresponding variations in the pressure of the working liquid and hence corresponding variations measured by the sensor unit.
During measurement of the interstitial pressure, the working liquid may contain air bubbles. These air bubbles may form when the piezocone is assembled and or they may be the result of non-saturation of the porous annular element or of cavitation phenomena in the working liquid inside the porous annular element E which may occur when the tip of the piezocone penetrates soils that produce a negative overpressure.
The air bubbles cause a certain amount of deformability in the mass of liquid contained in the sensor unit, in the sense that part of the interstitial overpressure to be measured, generated in the soil and acting on the lateral outside surface of the porous annular element, is used to compress the air bubbles. In this case, the liquid in the sensor unit becomes “compressible”, and in some cases to non-negligible levels.
At present, apparatuses are available which allow improving the saturation of the sensor unit by eliminating at least part of the air bubbles. Examples of such apparatuses are described in patent documents CN101858073 and CN204112301.
These apparatuses do not allow assessing the level or degree of saturation of the pressure sensor.